1. Field of the Invention
The present invention relates to a novel process for the separation by rectification of (meth)acrylic acid from a mixture containing (meth)acrylic acid and an inert hydrophobic organic liquid having a boiling point higher than that of (meth)acrylic acid as main constituents and also lower aldehydes as secondary constituents.
2. Discussion of the Background
(Meth)acrylic acid is used as an abbreviation and denotes acrylic acid or methacrylic acid.
(Meth)acrylic acid, either as such or in the form of its esters, is particularly important for preparing polymers for a very wide range of applications, eg. use as adhesives.
(Meth)acrylic acid can be obtained, inter alia, by catalytic gas-phase oxidation of alkanes, alkanols, alkenes or alkenals containing 3 or 4 carbon atoms. It can be particularly advantageously obtained, for example, by catalytic gas-phase oxidation of propene, acrolein, tert-butanol, iso-butene, iso-butane, iso-butyraldehyde or methacrolein. However, other possible starting compounds are those from which the actual C.sub.3 /C.sub.4 starting compound is only formed as an intermediate during the gas-phase oxidation. An example which may be mentioned is the methyl ether of tert-butanol.
These starting gases, generally diluted with inert gases such as nitrogen, CO, CO.sub.2, saturated hydrocarbons and/or steam, are passed in admixture with oxygen at elevated temperatures (usually from 200 to 400.degree. C.) and, if desired, superatmospheric pressure over transition metal mixed oxide catalysts (eg. containing Mo, V, W and/or Fe) and converted by oxidation into (meth)acrylic acid (cf., for example, DE-A 4 405 059, EP-A 253 409, EP-A 92 097, DE-A 44 31 957 and DE-A 44 31 949).
However, owing to the numerous parallel and subsequent reactions occurring in the course of the catalytic gas-phase oxidation, and also because of the inert diluent gases used, the product is not pure (meth)acrylic acid but rather a reaction mixture which contains essentially (meth)acrylic acid, the inert diluent gases and byproducts, from which the (meth)acrylic acid has to be separated. Besides byproducts which are comparatively simple to remove from (meth)acrylic acid and cause relatively little interference in subsequent use of the (meth)acrylic acid, such as acetic acid, the reaction mixture also contains, in particular, lower aldehydes which are closely related to (meth)acrylic acid and are therefore difficult to separate from (meth)acrylic acid, such as formaldehyde, acetaldehyde, acrolein, methacrolein, propionaldehyde, n-butyraldehyde, benzaldehyde, furfural and crotonaldehyde and possibly also maleic anhydride (based on the amount of (meth)acrylic acid present in the reaction gas mixture, the total amount of these secondary components which frequently cause considerable interference in subsequent use is generally &lt;2% by weight).
DE-A 44 36 243 relates to a process for separating (meth)acrylic acid from the reaction gas mixture of the catalytic gas-phase oxidation by countercurrent absorption using a high-boiling inert hydrophobic organic liquid, in which the reaction gas mixture is passed through an absorption column in countercurrent to the descending high-boiling inert hydrophobic organic liquid, a rectification process is superimposed on the absorption process occurring naturally in the absorption column, by withdrawing from the absorption column an amount of energy greater than its natural energy loss resulting from its contact with the surroundings, and the (meth)acrylic acid is separated by rectification from the liquid discharge from the absorption column (absorbate), which contains (meth)acrylic acid and the absorbant as main constituents and lower aldehydes and, possibly, maleic anhydride as secondary constituents. The (meth)acrylic acid obtainable in this way is described as crude (meth)acrylic acid. It generally has a purity &gt;98% by weight, with the impurities coming, in particular, from among the specified lower aldehydes and possibly maleic anhydride, while the separation of the (meth)acrylic acid from the high-boiling inert organic absorption liquid is essentially quantitative.
DE-A 44 36 243 defines high-boiling inert hydrophobic organic liquids (absorbants) as all those liquids whose boiling point at atmospheric pressure is above the boiling point of (meth)acrylic acid and which comprise at least 70% by weight of those molecules which contain no outward-acting polar group and are thus, for example, not able to form hydrogen bonds. This definition also applies here.
DE-C 2 136 396 and DE-A 43 08 087 likewise disclose the separation of acrylic acid from the reaction gas mixture of the catalytic gas-phase oxidation of propylene and/or acrolein by countercurrent absorption using a high-boiling inert hydrophobic organic liquid. The process is carried out essentially by passing the reaction gas mixture through a conventional absorption column in countercurrent to the descending absorption liquid, then, in a desorption column, largely removing the easily separable, readily volatile secondary components from the liquid discharge of the absorption column, composed of acrylic acid, the absorbant and secondary components, by stripping with inert gas, and subsequently treating by rectification the liquid discharge of the desorption column, which contains (meth)acrylic acid and the absorbant as main constituents and lower aldehydes and possibly maleic anhydride as secondary constituents, to separate off crude acrylic acid.
However, a disadvantage of the separation by rectification of crude (meth)acrylic acid from mixtures containing (meth)acrylic acid and a high-boiling inert hydrophobic organic liquid as main constituents and lower aldehydes and possibly maleic anhydride as secondary constituents is that during the course of rectification, despite use of amounts which are customary per se of customary polymerization inhibitors such as phenothiazine, paramethoxyphenol, paranitrosophenol, hydroquinone, hydroquinone monomethyl ether or air, the rectification apparatus (in particular the vaporizer surface and the internal fixtures of the column) become covered with a deposit. If the separation by rectification is operated continuously, the different coloration of the deposit in the stripping column (black) and the rectifying column (white) demonstrates that at least two processes participate in deposit formation. The formation of this deposit is disadvantageous because it has to be removed from time to time, which requires a shutting down of the rectification operation.
It is an object of the present invention to provide a process for the separation by rectification of (meth)acrylic acid from a mixture containing (meth)acrylic acid and an inert hydrophobic organic liquid having a boiling point higher than that of (meth)acrylic acid as main constituents and lower aldehydes as secondary constituents, which process makes possible reduced deposit formation and thereby extended rectification operation.